Process and device for adjusting the braking effect in a vehicle

ABSTRACT

The braking effect in a vehicle is set in the sense of preventing a tendency to lock-up with at least two wheels which are arranged on at least one axle on the right and left and whose motion characteristics are detected. The braking effect on the two wheels is set jointly, with at least two operating modes (select low, select high) being provided; they can be selected depending on the coefficients of friction prevailing on the wheels and/or depending on the vehicle speed. An operating mode is selected according to the value of the prevailing coefficients of friction when there are coefficient of friction differences of a preselectable lower value. As an alternative or in addition, an operating mode is selected according to the detected vehicle speed when there are coefficient of friction differences of a preselectable greater value.

FIELD OF THE INVENTION

The present invention relates to a method and a device for adjusting thebraking effect in a motor vehicle.

BACKGROUND INFORMATION

In the related art there are various known options for counteracting thetendency of a braked wheel to lock-up, for example, by manipulating thebraking force on each wheel using individual control. However, for suchindividual control, a method of controlling braking effect on each wheelindividually, i.e., separately from the other wheels should be provided.In the case of hydraulic brakes, such systems provide a control channelwith the corresponding hydraulic components for each wheel brake to becontrolled individually. Reference is made here to

Bosch Technische Berichte, Vol. 7, No. 2, 1980 for anti-lock systems asan example.

In addition to such brake systems, however, it is also known that thebraking effect on multiple wheels of a motor vehicle can be regulated orcontrolled jointly. German Patent No. 22 43 260 (corresponding to U.S.Pat. No. 3,907,378), for example, describes how the braking effect onthe wheel brakes of the wheels of one axle can be adjusted jointly inthe sense of preventing brake locking. Since the wheels on one axle maybe traveling on parts of the road surface having extremely differentroad surface/tire coefficients of friction, it must be determined whichof the wheels that are to be controlled jointly determines the brakingeffect. In this connection, it is known that two operating modes can beprovided for jointly setting the braking effect on two wheels of oneaxle.

In one operating mode, the select low mode, joint setting of the brakingeffect, i.e., the brake pressure in general, is based on the wheel withthe lower road surface/tire coefficient of friction. Thus, in the selectlow mode on one axle, the brake pressure on the wheel brakes of thisaxle is set so that the low wheel, i.e., the wheel with the lower roadsurface/tire coefficient of friction of the two wheels, is operated atoptimum slip. Consequently, the high wheel, i.e., the wheel with thehigher road surface/tire coefficient of friction, is underbraked and isstable (no excessive wheel slip), but then it can also withstand highlateral traction forces.

In the select high mode, the joint setting of the braking effect, or, ingeneral, the brake pressure, is activated according to the wheel withthe higher road surface/tire coefficient of friction. Thus, in theselect high mode on one axle, the brake pressure on the wheel brakes ofthis axle is set so that the wheel with the higher road surface/tirecoefficient of friction of the two wheels is operated at optimum slip.Consequently, the wheel with the lower road surface/tire coefficient offriction is overbraked and may even lock-up and then can withstandlittle or no lateral traction forces. The advantage of the select highmode, however, is that the vehicle has a relatively short brakingdistance.

German Patent No. 22 43 260 describes how the period of time duringwhich the wheel is switched to select high mode is to be made to dependon the vehicle speed and/or deceleration so that the time is longer at alower vehicle speed and/or deceleration.

Likewise, U.S. Pat. No. 3,535,004, which is cited in German Patent No.22 43 260, describes how switching to select high mode is to beperformed whenever the vehicle is traveling on a road surface with acoefficient of friction that differs significantly on the right and leftsides of the vehicle.

In addition, it is known that with a two-channel ABS (braking effects onthe front and rear axles are regulated jointly) or a three-channel ABS(braking effects on the rear axle are regulated jointly, but anindividual setting of the braking effect is possible on the front axle),the braking effect on the rear axle is set for select low mode,.

SUMMARY OF THE INVENTION

The object of the present invention is to provide for a brake systemwith a joint setting of the braking effect on the wheels of at least oneaxle so as to achieve the greatest possible driving safety.

The present invention concerns a system for setting the braking effectin a vehicle in the sense of avoiding the tendency to lock-up, with atleast two wheels which are arranged on an axle on the right and leftsides with respect to the direction of travel and which travel on a roadsurface having different coefficients of friction, and whose motioncharacteristics are detected. The braking effect on the two wheels isset jointly, with two operating modes being provided, which can beselected depending on the prevailing coefficients of friction on thewheels and/or depending on the vehicle speed. In the first selectableoperating mode (select low), the braking effect is jointly set dependingon the motion characteristics of the wheel having the lower coefficientof friction, while in a second selectable operating mode (select high)the joint setting is based on the motion characteristics of the wheelhaving the higher coefficient of friction. According to presentinvention, the coefficients of friction of the wheels on one axle aredetermined, and then the difference between the coefficients of frictionon the wheels on the right and left sides of the axle with respect tothe direction of travel is determined. When differences between thecoefficients of friction reach a predeterminable lower value, anoperating mode is selected according to the prevailing coefficients offriction. As an alternative or in addition, an operating mode may beselected according to the detected vehicle speed when the coefficient offriction differences reach a predeterminable greater value.

Thus, the difference in instantaneous coefficients of friction of theroad surface on the right and left sides of the vehicle is determined.If this difference is below a predetermined threshold, an operating modeis selected according to the prevailing coefficients of friction. As analternative or in addition, an operating mode may be selected accordingto the detected vehicle speed when the threshold is exceeded.

Especially when used in a two-channel brake system, the presentinvention yields a brake system with a simple but inexpensive designthat increases driving safety.

In particular, the present invention provides for the second operatingmode (select high) to be chosen, in particular for the front axle of thevehicle, when there are low coefficients of friction and the coefficientof friction difference is small. With this exemplary of the presentinvention, locking of wheels, in particular a front wheel, is acceptableunder some circumstances in order to achieve a short braking distance.However, the rear wheels should advantageously be operated in select lowmode to ensure optimum lateral traction. In principle, however, the rearwheels can also be operated in the select high mode.

The first operating mode (select low) is advantageously selected, inparticular for the front axle of the vehicle, when the coefficients offriction are high and the coefficient of friction difference is small.Because of the high coefficients of friction, good lateral traction ofthe wheels should not be sacrificed in this case.

Another advantageous exemplary calls for the second operating mode(select high) to be selected, in particular for the front axle of thevehicle, when the vehicle speed is low and the coefficient of frictiondifference is high. When vehicle speed is high and the coefficient offriction difference is great, the first operating mode (select low) isselected, in particular for the front axle of the vehicle. Thisexemplary is based on the consideration that at lower vehicle speeds, anon-optimal lateral traction of the front wheel with the lowercoefficient of friction can be accepted under some circumstances tooptimize the braking distance. At higher vehicle speeds, however, theselect low mode should be set on the front wheels to increase thelateral traction of the front wheels and thus improve steerability. Inthis exemplary, however, the rear wheels should also be operated in theselect low mode regardless of vehicle speed to ensure optimum lateraltraction of the rear wheels and thus stable vehicle performance. Inprinciple, however, the rear wheels may also be operated in the selecthigh mode.

Coefficient of friction differences of a larger and/or smaller value canbe detected by

detecting the rotational motion of each wheel,

determining the coefficients of friction on the wheels depending atleast on the detected wheel motion,

comparing the difference between the coefficients of friction determinedon the right and left wheels with preselectable threshold values.

The wheel deceleration is determined in this case from the rotationalmotion thus detected. The coefficient of friction can be deduced fromthe wheel deceleration, in particular the maximum wheel deceleration.

One advantageous exemplary of the present invention calls for the secondoperating mode (select high) to be selected in particular for both axlesof the vehicle when coefficient of friction differences are very great.This exemplary is based on the consideration that unacceptably longbraking distances can occur due to the select low mode when coefficientof friction differences are extremely different.

The selection of an operating mode can be provided with a hysteresisaccording to the present invention, so that there are not too manyswitches (pendulum switches) between operating modes at low coefficientof friction differences.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic block diagram of a two-channel brake system.

FIG. 2 shows a block diagram of a control unit that controls the brakesystem.

FIG. 3 shows switching between the two operating modes according to thepresent invention.

DETAILED DESCRIPTION

The vehicular hydraulic brake system according to the block diagramshown in FIG. 1 has a double-circuit main brake cylinder 117 with areservoir 104 and with a brake pedal 106 which is connected to a brakebooster 105 by a pedal stem. Two brake circuits HZ2 and HZ1 are providedfor brakes 120 hl and 120 hr of the rear axle of a vehicle and 120 vland 120 vr of the front axle.

Brake circuit HZ1 includes a main brake line 108 going from main brakecylinder 117 and leading to anti-lock device 10, and a wheel brake line118 v going from anti-lock device 10 and leading to wheel brakes 120 vland 120 vr of the front axle. Similarly, brake circuit HZ2 includes amain brake line 107 and a wheel brake line 118 h leading to wheel brakes120 hl and 120 hr of the rear axle.

Anti-lock device 10 has two valves MV1 and MV2 116 and 115 which areinstalled in series with two nonreturn valves 114 and 113. In addition,there are two recirculating pumps RFP1 and RFP2 112 and 111 which sharea drive motor (not shown). To reduce pressure surges, fluid isrecirculated into main brake lines 107, 108 through two damper chamberswith downstream throttles D2 and DI (109, 110).

Valves 115 and 116 are designed as 2/2-way valves which can becontrolled by electromagnets (control signals A1 and A2) and are openwhen there is no current through the electromagnets. Depending on theposition of solenoid valves 115 and 116, the brake pressure can be builtup or reduced jointly for the wheel brakes of the front axle and jointlyfor the wheel brakes of the rear axle. Reference numbers 130 hl, 130 hr,130 vl and 130 vr denote the wheel rpm sensors, which are essentiallyknown and detect the rotational speeds of the respective wheels andtransmit them to control unit 140 in the form of signals Nhl, Nhr, Nvland Nvr. Depending on these input signals, control unit 140 calculatescontrol signals A1 and A2 for driving solenoid valves 115 and 116.

The operation of control unit 140 will be explained in is more detailwith reference to FIG. 2.

In control unit 140, wheel rpm signals Nhl, Nhr, Nvl and Nvr are sent toblock 1403 in addition to being sent to block 1401 just described. Areference speed representing the vehicle's longitudinal speed is firstformed there from wheel rpm signals Nhl, Nhr, Nvl and Nvr. This can beaccomplished in an extremely simple manner for the brake control byusing the second largest wheel rpm as the reference speed (because thevehicle may be fitted with an emergency wheel with a smaller wheelradius).

In block 1403 the brake slip for each wheel is determined from thereference speed and the wheel rpm. An instability value is formed fromthe wheel slip thus determined and the wheel deceleration, optionallytaking into account other parameters. The braking stability of eachwheel can be deduced from the size of this instability value. In thecase of the above-mentioned individual control, the brake pressures ineach wheel brake would be controlled in such a way that stable wheelmotion is achieved, i.e., there is no excessive brake slip inparticular.

Control unit 140 causes the drive motor (not shown) to be switched onand thus recirculation pumps RFP1 and RFP2 to be started, depending onthe detection of a risk of wheel locking on at least one of the wheels.In addition, control unit 140 causes the closing of at least the one ofvalves MV1/MV2 whose respective wheel is at risk of wheel locking. Inthis way, because valve MV1/2 is closed, hydraulic medium is conveyedback to main brake cylinder 117 from the wheel brake cylinder byrecirculation pump RFP1/2, which is in operation. Consequently, thebrake pressure in the wheel brake cylinder drops, the braking effectdeclines and the risk of wheel locking is reduced. If the risk of wheellocking is reduced to a sufficient extent, the corresponding valve MV1/2is opened, so that it again assumes its starting position, with theresult that a pressure difference between the main brake cylinder andthe wheel brake cylinder is smaller due to the preceding reduction inbrake pressure.

Because of the limited hydraulic design, only a joint brake pressurecontrol on the two axles is possible, so that, as described in thepreamble, the brake pressure on the axles is set so that either thestability of the wheel with the lower road surface/tire coefficient offriction is given preference in the select low mode or the stability ofthe wheel with the higher road surface/tire coefficient of friction isgiven preference in the select high mode. To differentiate which of thetwo modes is to be set on which axle, both signal (SL/SH)_(VA) andsignal (SL/SH)_(HA) are sent from block 1402 to block 1403. Thesesignals can each assume two values, depending on whether

the select low mode (signal SL_(VA)) is to be set on the front axle,

the select high mode (signal SH_(VA)) is to be set on the front axle,

the select low mode (signal SL_(HA)) is to be set on the rear axle or

the select high mode (signal SH_(HA)) is to be set on the rear axle.

To determine which operating mode is to be set at a given moment, wheelrpm signals Nhl, Nhr, Nvl and Nvr are sent to block 1401. In block 1401the wheel deceleration values are calculated by differentiation. Sincethe maximum possible wheel deceleration is a direct measure of theinstantaneous coefficient of friction between tire and road surface,coefficient of friction μr for the right half of the vehicle andcoefficient of friction μl for the left half of the vehicle can bedetermined from the maximum wheel deceleration of the right and leftwheels. These values are sent to block 1402.

In block 1402, coefficients of friction μl and μr are compared with oneanother, whereupon signals (SL/SH)_(VA) and (SL/SH)_(HA) are formedaccording to the results of this comparison, the size of thecoefficients of friction and optionally instantaneous speed V of thevehicle. The diagram in FIG. 3 illustrates the formation of thesesignals.

FIG. 3 shows using a map the selection of operating modes according tocoefficients of friction μr and μl, on the right and left sides of thevehicle. There are no significant differences between the coefficientsof friction in the area of line 0, and the vehicle is traveling on ahomogeneous road surface. The differences between coefficients offriction μr and μl on the right and left sides of the vehicle are withincertain limits in the area between lines A1 and A2. Thus, in this areathe road surface may be nonhomogeneous to a limited extent. If theinstantaneous coefficients of friction are in this range (between A1 andA2) and if these coefficients of friction are not too high (range belowline C), the wheel brakes of the front axle are operated in the selecthigh mode and those of the rear axle are operated in the select low modethrough signals SH_(VA) and SL_(HA) delivered. In this range, locking ofa front wheel that may occur under some circumstances is acceptable toachieve a short braking distance. The anti-lock control on the rearwheels, however, is operated in the select low mode to ensure optimumlateral traction for stable driving performance.

However, if the coefficients of friction assume higher values in therange between A1 and A2 (above line C), the select low mode is selectedon both axles. Because of the high coefficients of friction, goodlateral traction of all wheels should not be sacrificed in this case.

If coefficients of friction μr and μl on the right and left sides of thevehicle differ significantly (range between lines B1 and A1 or A2 andB2), the mode is selected as a function of longitudinal speed V of thevehicle. The reference speed of the vehicle, which is needed in block1402 to determine the brake slip, can be sent to block 1402 as vehiclelongitudinal speed V.

If vehicle longitudinal speed V is equal to or lower than a selectablethreshold SW, the select high mode is selected for the wheel brakes ofthe front axle of the vehicle in the range between lines B1 and A1 or A2and B2. In the range of lower speeds, the shorter braking distance maybe given a higher priority than lateral traction of the low wheel.

However, if vehicle speed V is greater than a selectable threshold SW,the select low mode is selected for the wheel brakes of the front axleof the vehicle in the range between lines B1 and A1 or A2 and B2. In thehigher speed range, the lateral traction of the low wheel is thusassigned a higher priority than a shorter braking distance.

Regardless of the vehicle speed, however, the anti-lock control on therear wheels is operated in the select low mode in the range between A1and B1 or A2 and B2 to ensure optimum lateral stability.

If the coefficient of friction differences are great (range above B1 andbelow B2), the select high mode is chosen on the rear axle as well asthe front axle. This is due to the fact that when there is a greatdifference in coefficients of friction on the right and left sides ofthe vehicle, select low mode operation of the anti-lock system wouldlead to a very long braking distance because the wheel with the highcoefficient of friction, the high wheel, would always be severelyunderbraked.

In the exemplary illustrated with reference to FIG. 3, the operatingmodes of the anti-lock system are selected for the front and rear axlesaccording to the coefficients of friction μr and μl on the right andleft sides of the vehicle. It is also possible for the choice ofoperating modes to be made for each axle separately according to thecoefficient of friction differences on the respective axle or dependingon the prevailing vehicle speed.

In addition, the switching between operating modes as a function ofcoefficient of friction and vehicle speed may also be provided with ahysteresis. An unreasonably high number of switches (pendulum switches)between operating modes is avoided with such a hysteresis when there areonly slight fluctuations in coefficient of friction differences andslight fluctuations in vehicle speed around the threshold SW.

What is claimed is:
 1. A method for controlling a braking effect in avehicle to prevent a wheel lock-up tendency, the vehicle including atleast one axle and wheels rolling on a road surface, at least one rightwheel of the wheels being arranged on a right end of the at least oneaxle and at least one left wheel of the wheels being arranged on a leftend of the at least one axle, the at least one right wheel having afirst friction coefficient and the at least one left wheel having asecond friction coefficient, the first friction coefficient beingdifferent from the second friction coefficient, the method comprisingthe steps of: detecting first and second motion characteristics of thewheels; comparing the first friction coefficient to the second frictioncoefficient to determine a higher coefficient, a lower coefficient and adifference value between the first friction coefficient and the secondfriction coefficient; setting the braking effect on the wheels with ajoint setting device using a first operating mode and a second operatingmode, wherein, in the first operating mode, the braking effect is set asa function of the first motion characteristics of a first wheel of thewheels which has the lower coefficient, and wherein, in the secondoperating mode, the braking effect is set as a function of the secondmotion characteristics of a second wheel of the wheels which has thehigher coefficient; and selecting one of the first and second operatingmodes as a function of: one of the first and second frictioncoefficients when the difference value is smaller than a predeterminedvalue, and a speed of the vehicle when the difference value is greaterthan the predetermined value.
 2. The method according to claim 1,wherein a magnitude of the difference value relative to thepredetermined value is determined by: determining a rotational motion ofthe wheels, determining the higher coefficient at least as a function ofdefined characteristics of the first and second motion characteristics,and comparing the difference value to [preselected threshold values] thepredetermined value.
 3. The method according to claim 2, wherein thedefined characteristics are indicative of a maximum wheel decelerationdetermined from the rotational motion.
 4. The method according to claim1, further comprising the step of: selecting the second operating modefor a front axle of the at least one axle and a rear axle of the atleast one axle.
 5. The method according to claim 4, wherein the secondoperating mode is selected for a front axle of the at least one axle anda rear axle of the at least one axle when the difference value is equalto a high predetermined value.
 6. The method according to claim 1,further comprising the step of: selecting one of the first and secondoperating modes with hystereses.
 7. A method for controlling a brakingeffect in a vehicle to prevent a wheel lock-up tendency, the vehicleincluding at least one axle and wheels rolling on a road surface, atleast one right wheel of the wheels being arranged on a right end of theat least one axle and at least one left wheel of the wheels beingarranged on a left end of the at least one axle, the at least one rightwheel having a first friction coefficient and the at least one leftwheel having a second friction coefficient, the first frictioncoefficient being different from the second friction coefficient, themethod comprising the steps of: detecting first and second motioncharacteristics of the wheels; comparing the first friction coefficientto the second friction coefficient to determine a higher coefficient, alower coefficient and a difference value between the first frictioncoefficient and the second friction coefficient; setting the brakingeffect on the wheels with a joint setting device using a first operatingmode and a second operating mode, wherein, in the first operating mode,the braking effect is set as a function of the first motioncharacteristics of a first wheel of the wheels which has the lowercoefficient, and wherein, in the second operating mode, the brakingeffect is set as a function of the second motion characteristics of asecond wheel of the wheels which has the higher coefficient; selectingone of the first and second operating modes as a function of at leastone of: one of the first and second friction coefficients when thedifference value is smaller than a predetermined value, and a speed ofthe vehicle when the difference value is greater than the predeterminedvalue; selecting the second operating mode when the difference value issmaller than the predetermined value; and selecting the first operatingmode when the difference value is greater than the predetermined value.8. The method according to claim 7, wherein the first and secondoperating modes are selected for a front axle of the at least one axle.9. A method for controlling a braking effect in a vehicle to prevent awheel lock-up tendency, the vehicle including at least one axle andwheels rolling on a road surface, at least one right wheel of the wheelsbeing arranged on a right end of the at least one axle and at least oneleft wheel of the wheels being arranged on a left end of the at leastone axle, the at least one right wheel having a first frictioncoefficient and the at least one left wheel having a second frictioncoefficient, the first friction coefficient being different from thesecond friction coefficient, the method comprising the steps of:detecting first and second motion characteristics of the wheels;comparing the first friction coefficient to the second frictioncoefficient to determine a higher coefficient, a lower coefficient and adifference value between the first friction coefficient and the secondfriction coefficient; setting the braking effect on the wheels with ajoint setting device using a first operating mode and a second operatingmode, wherein, in the first operating mode, the braking effect is set asa function of the first motion characteristics of a first wheel of thewheels which has the lower coefficient, and wherein, in the secondoperating mode, the braking effect is set as a function of the secondmotion characteristics of a second wheel of the wheels which has thehigher coefficient; selecting one of the first and second operatingmodes as a function of at least one of: one of the first and secondfriction coefficients when the difference value is smaller than apredetermined value, and a speed of the vehicle when the differencevalue is greater than the predetermined value; selecting the secondoperating mode when the speed is smaller than a predetermined speedvalue and the difference value is greater than the predetermined value;and selecting the first operating mode when the speed is greater thanthe predetermined speed value and the difference value is greater thanthe predetermined value.
 10. The method according to claim 9, whereinthe first and second operating modes are selected for a front axle ofthe at least one axle.
 11. A device for controlling a braking effect ina motor vehicle, the motor vehicle including at least one axle andwheels, at least one right wheel of the wheels being arranged on a rightend of the at least one axle and at least one left wheel of the wheelsbeing arranged on a left end of the at least one axle, the at least oneright wheel having a first friction coefficient and the at least oneleft wheel having a second friction coefficient, the first frictioncoefficient being different from the second friction coefficient, thedevice comprising: a first arrangement detecting motion characteristicsof the wheels; a second arrangement jointly setting the braking effecton the wheels; and a control arrangement generating control signals forcontrolling the second arrangement using at least two operating modes,the control arrangement determining a higher coefficient, a lowercoefficient and a difference value between the first frictioncoefficient and the second friction coefficient by comparing the firstfriction coefficient to the second friction coefficient, the controlarrangement selecting one of the at least two operating modes as afunction of: one of the first and second friction coefficients when thedifference value is smaller than a predetermined value, and a speed ofthe motor vehicle when the difference value is greater than thepredetermined value, wherein, in a first operating mode of the at leasttwo operating modes, the second arrangement jointly sets the brakingeffect as a function of the motion characteristics of a first wheel ofthe wheels which has the lower coefficient, and wherein, in a secondoperating mode of the at least two operating modes, the secondarrangement jointly sets the braking effect as a function of the motioncharacteristics of a second wheel of the wheels which has the highercoefficient.
 12. The device according to claim 11, wherein thedifference value is determined by: a third arrangement detecting arotational motion of each of the wheels, a fourth arrangementdetermining the higher coefficient at least as a function of the motioncharacteristics, and a fifth arrangement comparing the difference valueto predetermined threshold values.
 13. The device according to claim 12,wherein the motion characteristics are indicative of a function of amaximum wheel deceleration determined from the rotational motion.
 14. Adevice for controlling a braking effect in a motor vehicle, the motorvehicle including at least one axle and wheels, at least one right wheelof the wheels being arranged on a right end of the at least one axle andat least one left wheel of the wheels being arranged on a left end ofthe at least one axle, the at least one right wheel having a firstfriction coefficient and the at least one left wheel having a secondfriction coefficient, the first friction coefficient being differentfrom the second friction coefficient, the device comprising: a firstarrangement detecting motion characteristics of the wheels; a secondarrangement jointly setting the braking effect on the wheels; and acontrol arrangement generating control signals for controlling thesecond arrangement using at least two operating modes, the controlarrangement determining a higher coefficient, a lower coefficient and adifference value between the first friction coefficient and the secondfriction coefficient by comparing the first friction coefficient to thesecond friction coefficient, the control arrangement selecting one ofthe at least two operating modes as a function of at least one of: oneof the first and second friction coefficients when the difference valueis smaller than a predetermined value, and a speed of the motor vehiclewhen the difference value is greater than the predetermined value,wherein, in a first operating mode of the at least two operating modes,the second arrangement jointly sets the braking effect as a function ofthe motion characteristics of a first wheel of the wheels which has thelower coefficient, wherein, in a second operating mode of the at leasttwo operating modes, the second arrangement jointly sets the brakingeffect as a function of the motion characteristics of a second wheel ofthe wheels which has the higher coefficient, and wherein the controlarrangement selects the second operating mode for the lower coefficientwhen the difference value is smaller than the predetermined value, andwherein the control arrangement selects the first operating mode forhigher coefficient when the difference value is smaller than thepredetermined value.
 15. The device according to claim 14, wherein thecontrol arrangement selects the first and second operating modes for afront axle of the at least one axle.
 16. A device for controlling abraking effect in a motor vehicle, the motor vehicle including at leastone axle and wheels, at least one right wheel of the wheels beingarranged on a right end of the at least one axle and at least one leftwheel of the wheels being arranged on a left end of the at least oneaxle, the at least one right wheel having a first friction coefficientand the at least one left wheel having a second friction coefficient,the first friction coefficient being different from the second frictioncoefficient, the device comprising: a first arrangement detecting motioncharacteristics of the wheels; a second arrangement jointly setting thebraking effect on the wheels; and a control arrangement generatingcontrol signals for controlling the second arrangement using at leasttwo operating modes, the control arrangement determining a highercoefficient, a lower coefficient and a difference value between thefirst friction coefficient and the second friction coefficient bycomparing the first friction coefficient to the second frictioncoefficient, the control arrangement selecting one of the at least twooperating modes as a function of at least one of: one of the first andsecond friction coefficients when the difference value is smaller than apredetermined value, and a speed of the motor vehicle when thedifference value is greater than the predetermined value, wherein, in afirst operating mode of the at least two operating modes, the secondarrangement jointly sets the braking effect as a function of the motioncharacteristics of a first wheel of the wheels which has the lowercoefficient, wherein, in a second operating mode of the at least twooperating modes, the second arrangement jointly sets the braking effectas a function of the motion characteristics of a second wheel of thewheels which has the higher coefficient, and wherein the controlarrangement selects the second operating mode when the speed is smallerthan a predetermined speed value and when the difference value isgreater than the predetermined value, and wherein the controlarrangement selects the first operating mode when the speed is greaterthan the predetermined speed value and when the difference value isgreater than the predetermined value.
 17. The device according to claim16, wherein the control arrangement selects the first and secondoperating modes for a front axle of the at least one axle.